Ternary Ti-Ni-Cu Shape Memory Alloy and Process for Producing Same

ABSTRACT

An amorphous Ti—Ni—Cu alloy comprising from 44 to 49 atomic % of Ti, from 20 to 30 atomic % of Cu, and the balance being Ni and unavoidable elements is heated at  500  to 700° C. for a period of time not exceeding 100 hours to crystallize the amorphous alloy.

TECHNICAL FIELD

The present invention relates to a ternary Ti—Ni—Cu shape memory alloywhich has both low composition dependency and low heat treatmentdependency and which is useful as a device used, for example, in anactuator, and to a process for producing same.

BACKGROUND ART

A binary Ti—Ni alloy which is widely used as a shape memory alloy hasdefects because its phase transformation temperature greatly dependsupon its composition and its heat treatment temperature and is lowerthan ambient temperature when a large output force is attempted to beobtained. Thus, a difficulty is encountered in controlling thecomposition. In particular, the yield of sputtered thin films in whichthe compositional distribution in the plane direction unavoidablybecomes non-uniform is poor. It is, therefore, difficult to producesputtered films on an industrial scale (Patent Document 1).

With a view toward solving the above problems of such a binary Ti—Nialloy, studies have been made on a ternary Ti—Ni—Cu shape memory alloyin which a part of Ni of a 50 atomic % Ti—Ni alloy is substituted withCu. For example, it has been revealed that in Ti—Ni—Cu alloy thin filmshaving a Ti content of at least 50 atomic %, the temperature hysteresisis reduced by addition of Cu and the recovery stress increases due tosolid solution hardening by Cu (Non-Patent Document 1). It has been alsorevealed that in Ti—Ni—Cu alloy thin films containing 6 atomic % of Cuand no more than 50 atomic % of Ti, the shape memory behavior of alloyshaving a structure in which a TiNiCu phase is formed within grainsgreatly varies with a Ti content and, further, the transformation occursin a temperature range lower than ambient temperature and in twoseparate stages (Non-Patent Document 2).

A Ti50-(Ni, Cu)50 alloy which is a ternary Ti—Ni—Cu shape memory alloy,however, is brittle and has poor workability, though its phasetransformation temperature scarcely depends upon the Cu content. Thus,the Cu content is at most 10 atomic % in the case of a cast alloy and isat most 20 atomic % in the case of an alloy formed by a liquid quenchingmethod. Additionally, the obtained alloy has a composition near theTi(Ni, Cu) single phase (Ti 50 atomic %) and is defective in that theoutput force is small. For example, there is proposed a method ofproducing a Ti—Ni—Cu alloy which contains more than 10 atomic % of Cuand which is difficult to be produced with the ordinary melting and hotprocessing method (Patent Document 2). Since the composition of theproduced alloy is limited to the single phase region in the vicinity ofTi-50 atomic %, however, the alloy has a defect that the output force issmall.

-   Patent Document 1: JP-B-2899682-   Patent Document 2: JP-A-H06-172886

Non-Patent Document 1

[Transformation and Deformation Behavior in Sputter-Deposited Ti—Ni—CuThin Films], T. Hashinaga, S. Miyazaid, T. Ueki and H. Hirokawa: J.Physique IV, 5(1995), C8-689

Non-Patent Document 2

“Structure Evolution in Sputtered Thin Films of Tix(Ni, Cu)1−x” [1:Diffusive transformations], [2: Displace Transformations], L. Chang andD. S. Grumman: Philosophical Magazine A 76(1997), 163-219

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

As described in the foregoing, since the conventional shape memory alloythin film are sensitive to composition variations and heat treatmentconditions, it has been difficult to properly control their compositionand heat treatment conditions. Further, with the customarily employedsputtering method, it has not been easy to obtain a uniform distributionof the alloy composition in the plane direction thereof and to producesuch films in a satisfactory yield. It has been, therefore, difficult toproduce shape memory alloy thin films on an industrial scale. Further,since various treatments for increasing the output force of a shapememory alloy tend to lower the transformation temperature and ductilityof the alloy, it has been difficult to produce a shape memory alloywhich has a high transformation temperature and a large output forceand, yet which has useful ductility.

In this circumstance, the objective of the present invention is toprovide a ternary Ti—Ni—Cu shape memory alloy which has solved theabove-described problems, which has low composition dependency, whichpermits stable production, which has a transformation temperature higherthan ambient temperature and which can generate a large output force,and to provide a process capable of producing such a ternary Ti—Ni—Cushape memory alloy in an efficient manner.

Means for Solving the Problems

In accomplishing the foregoing objects, there is provided in accordancewith a first aspect of the present invention a ternary Ti—Ni—Cu shapememory alloy comprising from 44 to 49 atomic % of Ti, from 20 to 30atomic % of Cu, and the balance being Ni and unavoidable elements.

In a second aspect, the present invention provides the above alloy,wherein a TiNiCu or TiCu phase of not greater than 500 nm is formedwithin Ti(Ni, Cu) crystal grains having a grain size of 2 μm or less.

In a third aspect, the present invention provides a process, whichcomprises beating an amorphous Ti—Ni—Cu alloy comprising from 44 to 49atomic % of Ti, from 20 to 30 atomic % of Cu, and the balance being Niand unavoidable elements to crystallize the amorphous Ti—Ni—Cu alloy.

In a fourth aspect, the present invention provides the above process,wherein said heating is at a temperature in the range of from 500 to700° C.

In a fifth aspect, the present invention provides the above process,wherein said heating is performed for a period of time not exceeding 100hours.

Effect of the Invention

The ternary Ti—Ni—Cu shape memory alloy according to the presentinvention has low composition or heat treatment dependency and atransformation temperature higher than ambient temperature and iscapable of being produced in a stable manner. The shape memory alloy isuseful as a device used, for example, in an actuator.

The present invention also provides a process capable of producing theabove ternary Ti—Ni—Cu shape memory alloy in an efficient manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows shape memory characteristics of a 48.3 Ti-27.8 Cu—Ni alloyobtained by heat treatment at 600° C. for 1 hour.

FIG. 2 is a microphotograph showing the structure of a 48.3 Ti-27.8Cu—Ni alloy obtained by heat treatment at 600° C. for 1 hour.

FIG. 3 shows Ti content dependence and Cu content dependence of theoutput force generated by a Ti—Ni—Cu alloy obtained by heat treatment at600° C. for 1 hour.

FIG. 4 shows Ti content dependence and Cu content dependence of themartens tic transformation temperature of a Ti—Ni—Cu alloy obtained bybeat treatment at 600° C. for 1 hour.

FIG. 5 shows heat treatment temperature dependence (for a heat treatmenttime of 1 hour) and heat treatment time dependence (for a heat treatmenttemperature of 600° C.) of the output force generated by Ti—Ni—Cu alloysof various compositions.

FIG. 6 shows heat treatment temperature dependence (for a heat treatmenttime of 1 hour) and heat treatment time dependence (for a heat treatmenttemperature of 600° C.) of the martensitic transformation temperature ofTi—Ni—Cu alloys of various compositions.

FIG. 7 shows heat treatment temperature dependence (for a heat treatmenttime of 1 hour) and heat treatment time dependence (for a heat treatmenttemperature of 600° C.) of the temperature hysteresis of Ti—Ni—Cu alloysof various compositions.

DESCRIPTION OF REFERENCE NUMERALS

-   1: grain boundary precipitates-   2: precipitates within grains

BEST MODE FOR CARRYING OUT THE INVENTION

In the studies on ternary Ti—Ni—Cu alloys of B 19 structure in which thechange in crystal structure is small, it has been found that a ternaryTi—N—Cu shape memory alloy exhibiting, in a stable maimer without beinginfluenced by the alloy composition or heat treatment conditions,transformation at a temperature higher than ambient temperature andcapable of generating a large output force can be obtained by adopting,in combination, enrichment of precipitates and solid solutionhardening-in addition to microsize of grains (suppressing the grain sizeup to about 2 μm) which alone would cause lowering of the transformationtemperature. The present invention has been completed based on thetechnical finding.

A large output force generated by the ternary Ti—Ni—Cu shape memoryalloy according to the present invention is considered to be ascribed tothe synergetic effect among the solid solution hardening by Cu atom, theprecipitation of a TiNiCu phase or a TiCu phase within the Ti(Ni, Cu)crystal grains, and the microsize of the Ti(Ni, Cu) crystal grains. Whenthe heat treatment is carried out at a high temperature for anexcessively long period of time, however, the size of the crystal grainsincreases and the formation of such precipitates within the crystalgrains decreases and, therefore, a high output force could not beobtained. For this reason, the heat treatment is preferably carried outat 500 to 700° C. for 100 hours or less.

In the process for producing a ternary Ti—Ni—Cu shape memory alloyaccording to the present invention, an amorphous Ti—Ni—Cu alloy composedof Ti in an amount of from 44 to 49 atomic %, Cu in an amount of from 20to 30 atomic %, and the balance being Ni and unavoidable elements isheated at a temperature from 500 to 700° C. for a period of time notexceeding 100 hours. The reasons for specifying the contents of theabove elements are as follows. When the Ti content exceeds 49 atomic %,a TiNiCu phase which is one of the factors to increase the output forceof the ternary Ti—Ni—Cu shape memory alloy is not formed. When the Ticontent is less than 44 atomic %, on the other hand, the TiNiCu phaseincreases excessively to cause not only lowering of the transformationtemperature but also brittleness of the alloy. Therefore, the Ti contentshould be within the range of from 44 to 49 atomic %. When the Cucontent exceeds 30 atomic %, only a TiCu phase is formed and the alloybecomes brittle. On the other hand, when the Cu content is less than 20atomic %, the transformation temperature is lowered so that it is nolonger possible to ensure a transformation temperature higher thanambient temperature (suitably 40° C. or higher) in a stable mannerthroughout the entire range of 49 to 44 atomic % of the Ti content.Further, a large output force cannot be obtained because the solidsolution hardening by Cu is insufficient and the grain size becomeslarge. Therefore, the Cu content should be within the range of from 20to 30 atomic %. The following example will further specificallyillustrate the ternary Ti—Ni—Cu shape memory alloy and its productionprocess.

EXAMPLE

Amorphous alloy thin films of 48.3 atomic % Ti-23.3 atomic % Cu—Ni; 48.3atomic % Ti-27.8 atomic % Cu—Ni; 44.6 atomic % Ti-23.2 atomic % Cu—Ni;and 44.9 atomic % Ti-27.3 atomic % Cu—Ni were produced using amulti-target magnetron sputtering system. Such amorphous thin films maybe produced not only by sputtering but also by using any other suitablemethod. Each of the amorphous thin films was peeled off from a substrateand subjected to a heat treatment at 500 to 700° C. The obtained alloyfilms were measured for their transformation temperature by differentialthermal analysis and for their shape memory characteristics by heatingand cooling under a loaded state.

FIG. 1 shows an example of the measured results of shape memorycharacteristics, from which it is seen that almost no residual strainremains upon cooling and heating under a high load stress and that thetransformation temperature is higher than ambient temperature.

FIG. 2 is an electron microphotograph showing the structure of one ofthe obtained ternary Ti—Cu—Ni shape memory alloys. Thermally stable,micro-size (500 nm or less) TiCu phase 1 or TiNiCu phase 2 is formedwithin the crystal grains or in the grain boundaries. The crystal grainsize is limited to not greater than 2 μm. As a result of synergism amongthe promotion of microsize crystal grains, the enrichment ofprecipitates within grains and the solid solution hardening by Cu, it ispossible to improve brittleness common to cast alloys and, at the sametime, to obtain a high resistance to plastic deformation which wouldcause residual strains.

FIG. 3 shows composition dependence of the output force generated by oneof the obtained ternary Ti—Cu—Ni shape memory alloys. The output forceis represented by the stress causing a residual strain of at least0.03%. It is seen that when the Cu content is 20 atomic % or higher, ahigh output force is obtained in a stable manner throughout thecomposition range of 49 to 44 atomic % Ti. It is also seen that, becauseof the formation of precipitates, the 48.3 atomic % Ti-23.3 atomic %Cu—Ni alloy shows a higher output force as compared with 50 atomic %Ti-23.3 atomic % Cu—Ni alloy and that, because of the solid solutionhardening by Cu, the 48.3 atomic % Ti-23.3 atomic % Cu—Ni alloy shows ahigher output force as compared with 48.3 atomic % Ti-11.5 atomic %Cu—Ni alloy. A TiNiCu phase and a TiCu phase precipitated in largeamounts when the Ti content was less than 44 atomic % and when the Cucontent exceeded 30 atomic %, respectively. Therefore, in either case,good shape memory alloys were not obtainable due to breakage.

FIG. 4 shows composition dependence of the transformation temperature(peak temperature). While ordinary shape memory alloys will cause areduction of the transformation temperature if the composition isaltered to obtain a high output force, a transformation temperaturehigher than ambient temperature is found to be obtained throughout thecomposition range of 49 to 45 atomic % Ti when the Cu content is in therange of 20 to 30 atomic %.

It has been confirmed that any of the above-described thin films of theternary Ti—Ni—Cu shape memory alloy forms a low temperature phase (B19phase) in the unloaded state at ambient temperature. FIG. 5 shows heattreatment dependence of the output force generated. It is appreciatedthat a high output force is obtainable in a wide range of heat treatmentconditions.

However, the output force tends to lower when heat treatment time isprolonged and the heat treatment temperature is increased. The heattreatment at 500 to 700° C. for 100 hours or less is found to bepreferred.

FIG. 6 shows beat treatment dependence of the martensitic transformationtemperature. All of the alloys had a martensitic transformationtemperature higher than ambient temperature. FIG. 7 shows thetemperature hysteresis of the ternary Ti—Ni—Cu alloys. The temperaturehysteresis of the transformation of any of the alloys is as small as 7to 15° C.

1-5. (canceled)
 6. A Ti—Ni—Cu shape memory alloy, containing Ti inamount of 44 atomic % to 49 atomic %, Cu in amount of 20 atomic % to 30atomic % and the rest consisting of Ni and inevitable impurities,wherein TiNiCu phases or TiCu phases sized 500 nm or less precipitate ina Ti(Ni,Cu) crystal grain whose grain size is 2 μm or less.
 7. A methodof producing a Ti—Ni—Cu shape memory alloy, wherein an amorphousTi—Ni—Cu alloy, which contains Ti in amount of 44 atomic % to 49 atomic%, Cu in amount of 20 atomic % to 30 atomic % and the rest consisting ofNi and inevitable impurities, is crystallized by heat treatment.
 8. Themethod of producing a Ti—Ni—Cu shape memory alloy as claimed in claim 7,wherein a temperature range of the heat treatment is 500° C. to 700° C.9. The method of producing a Ti—Ni—Cu shape memory alloy as claimed inclaim 7, wherein time of the heat treatment is not beyond 100 hours. 10.The method of producing a Ti—Ni—Cu shape memory alloy as claimed inclaim 8, wherein time of the heat treatment is not beyond 100 hours.